In general, the invention relates to gas hydrate-based desalination and/or water purification. In particular, the invention significantly reduces the amount of residual brine that mixes with the product water, thereby greatly enhancing the purity of the product water.
Purified water may be obtained from saline or polluted water by forming and then dissociating crystalline hydrate. Such a process for obtaining purified water from saline or polluted water is disclosed in, for example, U.S. Pat. Nos. 5,873,262 and 3,027,320. According to those patents, a gas or mixture of gases is brought into contact with saline or polluted water under appropriate conditions of pressure and temperature and forms hydrate. The hydrate is then brought to a region of higher temperature and lower pressure, where it dissociates to release fresh water and the hydrate-forming gas or gases.
When the hydrate is formed in saltwater to desalinate it, highly saline brines typically remain in the interstices of the hydrate as it forms a slurry. These brines may also contain dissolved or suspended solids.
One of the principal problems that has inhibited the successful development of hydrate-based desalination on a commercial scale has been the difficulty of removing such residual, interstitial brines from the hydrate slurry or a hydrate-brine mixture. In particular, it has proven difficult to develop a successful process for thoroughly washing an essentially static mixture of hydrate and interstitial brines, in which process the saline interstitial fluid is removed (and perhaps replaced by less saline interstitial water).
According to the two patents noted above, the hydrate, which is positively buoyant, simply floats upward from where it forms (a region of highly saline water) into a region of less saline water. The hydrate dissociates in the region of less saline water, while residual brine remains in or sinks toward the region of highly saline water. The region of less saline water may be maintained at the reduced salinity levels by introducing fresh water released upon dissociation of the hydrate. Such moving of hydrate, or allowing of hydrate to move, into a region of less saline water minimizes undesirable mixing of “purified” water with interstitial water and is particularly well suited to large-scale production of fresh water. However, variable amounts of highly saline residual fluid still enters the region of hydrate dissociation, which increases the salinity in the dissociation region and thus reduces the “purity” of the product water.
In addition to research on using hydrates for desalination/purification, much of the hydrate research to date has been conducted by energy companies concerned with inhibiting hydrate formation and growth in hydrocarbon pipelines because hydrate-caused flow constrictions in such pipelines can be extremely costly. Moreover, even if hydrate does not cause a flow constriction, small crystals of hydrate may form in petroleum, which crystals act as abrasive crystals in the moving fluid. Therefore, it is desirable to remove hydrate from pipelines and other hydrocarbon-containing vessels, even if the hydrate occurs only in small quantities.
Prior energy industry research efforts have yielded a number of methods for inhibiting hydrate growth or for removing unwanted hydrate from piping. However, existing methods involve high capital costs, high energy demands, and in some cases, the use of chemicals (such as alcohols) which absorb the water from petroleums but which create their own separation problems. If drying of petroleums is carried out on the seafloor in deep water, costs are magnified.